In optical communications, an optical clock multiplexer is used for multiplying a low-rate optical clock signal in an optical time division multiplexing (OTDM) system. By way of example, optical carrier wave pulses of 10 GHz are divided into two systems, and the optical carrier wave pulses of the respective systems are modulated according to data signals of 10 Gigabit/Second, for instance. One pulse wave of these modulated carrier waves is given a phase difference of half cycle (p) relative to the other pulse wave of the modulated carrier waves, and thereafter, these modulated carrier wave pulses are synthesized. According to such interleave processing, for example, an optical signal of 10 GHz is sent out as an optical signal of 20 Gigabit/Second.
In recent years, an optical clock (carrier wave) over 160 Gigabit/Second is coming to be considered, which is used for the optical time division multiplexing. Followings are extremely important as constituent elements of the optical clock multiplexer that is used in high-rate clock synthesis for the ultrafast optical time division multiplexing; smoothing of clock signal intensity after the synthesis, and equally spacing of clocks by correcting minor phase shifting that is caused by a change of temperature of the clock multiplexer main unit and externally connected equipment. Therefore, it becomes necessary to correct a peak value and a phase of the optical clock by using a spatial light modulator module.
FIG. 36 is an illustration to explain a configuration example which implements the optical clock multiplexer used for the optical time division multiplexing. In the configuration example as shown in FIG. 36, the optical clock multiplexer 100 is provided with an input port 102 and an output port 103. Optical signals inputted from the input port 102 are separated by an optical coupler/spectrometer 104. Some optical signals being separated are modulated by a light modulator 101, and the other optical signals are allowed to go through a fixed delay element 106, in which a delay time is fixed, and then both optical signals are coupled by the optical coupler/spectrometer 105. The optical signals coupled by the optical coupler/spectrometer 105 are outputted from the output port 103. Here in the light modulator 101, intensity of the optical signals and phase amount thereof are adjusted, thereby multiplying the optical clock that is used for the optical time division multiplexing.
FIG. 37 shows one example of the light modulator used for wavelength division multiplexing which employs liquid crystal elements. Patent document 1 is known as a disclosure of this type of light modulator, for instance. This light modulator is applied to R-OADM (Reconfigurable Add/Drop Multiplexer). It is to be noted that FIG. 37 illustrates a reflection type configuration example, wherein FIG. 37A shows the z-y plane, and FIG. 37B shows the x-z plane.
In the light modulator 200, a spectrometer 202 (a diffraction grating in this example) and an OPMC (optical phased matrix coupling) 203 are arranged on an optical path which is connected to an input/output port 201 for inputting an input signal beam including multiple wave lengths and for outputting an output signal beam, through cylindrical lenses 211, 212, and 213 which are prepared for forming parallel light.
[Patent Document 1]
    US Unexamined Patent Application Publication No. 2006/0067611A1[Patent Document 2]    Japanese Unexamined Patent Application Publication No. 6-51340 (paragraph 0010, 0017, and 0018)